Carbohydrates are an important element in the diet, and many of the foods that are rich in carbohydrates are also rich in fiber and phytonutrients. Good carbs are simply fruits, vegetables and some whole grains. Carbohydrate needs should be met first by consuming five to nine servings per day of diverse and colorful fruits and vegetables, which provide a wealth of beneficial substances. If more carbohydrates are needed, they can be supplied by whole grains and legumes (e.g., beans). A low-carbohydrate diet restricts carbohydrate grams to such a low level that individuals consuming these diets cannot benefit from the many health benefits of fruits and vegetables.
Carbohydrates are made up of atoms of carbon, hydrogen and oxygen but no nitrogen, and are broken down into sugars, such as glucose and fructose. Undigested carbohydrates are eliminated from the body and are referred to as dietary fibers.
Hidden Simple Sugars
Simple sugars include glucose, fructose, lactose and sucrose. These are listed as sugars on food labels, while the so-called complex carbohydrates are not included in this list, despite their similarity to simple sugars. Simple sugars can be directly absorbed by the mucous membranes of the mouth. However, short-chain carbohydrates, such as maltodextrin or corn sugar, consist of 15 glucose units, which are hydrolyzed in the stomach by enzymes and acid into simple sugars. These are often called complex carbohydrates on food labels but act like sugars.
The digestion of starches begins in the mouth in the presence of salivary amylase. As with proteins, much of the digestion takes place on the intestinal mucosal villi, which have both digestive enzymes and specific transport systems for sugars.
Lactose and sucrose are combinations of two different sugars linked together. Lactose is made up of galactose and glucose, while table sugar or sucrose consists of glucose and fructose.
High-fructose corn syrup is made from corn by a process that ends with 55 percent free fructose and 45 percent free glucose, approximately equivalent to table sugar. The tastes of corn sugar, sucrose and fructose are different. Fructose, the sweetest-tasting sugar, is found in fruits such as oranges. Corn sugar tastes like pancake syrup and is the primary sweetener in colas in the United States. In some countries, such as Mexico, sucrose is used to sweeten colas and they taste distinctly different from their U.S. counterparts. The issue with corn syrup is not its chemical character but the huge amount in the diet. Due to government subsidization of corn, large amounts of corn syrup are used in many foods,including soft drinks, which results in extra calories added to the diet. Studies suggest that the obesity epidemic can be linked to the following:
- Consumption of large amounts of high fructose corn syrup (HFCS).
- Consumption of high-fat foods.
- A sedentary lifestyle.
Since dietary fibers are not digested, they do not contribute directly to the nutritive value of foods in terms of calories, but they have many effects on human physiology. Ancient man consumed a great deal of fiber, and this fiber resulted in numerous large, bulky stools that filled the colon and caused it to contract against a large volume load. Modern man eats a small amount of fiber, approximately 10 to 15 grams per day, compared to 25 grams per day in a healthy, plant-based diet and well over 50 grams per day in ancient diets.
Carbohydrates have a bad reputation, due in part to the recent popularity of so-called high-protein, low-carbohydrate diets for weight loss. Classifying foods as carbs, proteins or fats is misleading since few foods are composed purely of one macronutrient, and the quality of the food can vary significantly. A high-carbohydrate diet could be a plant-based, whole-foods diet with phytonutrient-rich fruits and vegetables at the base, with a moderate amount of whole grains and healthy low-fat proteins to balance nutritional needs. But, since sugars, refined flour products (such as white bread and pasta) and refined grains (such as white rice) are all considered carbohydrates, a diet that is based primarily on refined grains, while it could be low in fat, could also be very high in calories because these low-fiber grain foods are not particularly filling.
Individuals who consume a diet of this type may feel virtuous for avoiding fat, but they could easily gain weight on a diet based on refined grains. The recent popularity of a high-protein, low-carbohydrate diet came on the tails of the high-carbohydrate craze of 20 years ago, because people found they were gaining weight on bread, cereal, rice and pasta if they made no distinction between whole grains and refined products. Pasta, which had previously been considered good because it is low in fat, is now viewed as bad because it is often a refined flour product (there are whole grain versions available).
In sports nutrition, especially for aerobic exercises, large amounts of carbohydrates are used to provide energy that is burned in the course of exercise. In the Fitness Science section, this topic is discussed in detail.
Recent studies in mice have indicated an interrelation between energy balance, diet, and the composition of the gut bacteria.
The Fitness Textbook will teach you the basic principles of human performance nutrition, including muscle metabolism and fuel utilization before, during and after exercise.